1. Field of the Invention
The invention relates to an electrode catalyst for a fuel cell, a method for producing the electrode catalyst, and a polymer electrolyte fuel cell using the electrode catalyst, which are developed so as to improve the performance of a platinum catalyst or platinum-alloy catalyst.
2. Description of the Related Art
The cell characteristics of polymer electrolyte fuel cells have remarkably improved for some reasons including the following reasons: 1) polymer electrolyte membranes having a high ion conductivity have been developed, and 2) catalyst-carrying carbon that is coated with a polymer electrolyte of the same type as or a different type from the polymer electrolyte membrane is used as a material that forms an electrode catalyst layer, so that three-dimensional reaction sites are provided in the catalyst layer. The polymer electrolyte fuel cells, which exhibit the improved cell performance, can be easily made compact and lightweight; therefore, it has been expected to put the polymer electrolyte fuel cells to practical use in mobile vehicles, such as electric automobiles, or as a power source of a compact cogeneration system, for example.
Generally, a gas-diffusible electrode used in a polymer electrolyte fuel cell consists of a catalyst layer containing the catalyst-carrying carbon coated with the polymer electrolyte as described above, and a gas diffusion layer that supplies reaction gas to the catalyst layer and collects electrons. In the catalyst layer, porous portions are present in which micropores are formed among secondary particles or tertiary particles of carbon as a constituent material of the catalyst layer, and the porous portions function as diffusion channels for the reaction gas. A catalyst formed of a noble metal, such as platinum or platinum alloy, which is stable in the polymer electrolyte is generally used as the above-mentioned catalyst.
For example, a catalyst having a noble metal, such as platinum or platinum alloy, supported on carbon black has been used as cathode and anode catalysts (electrode catalysts) of polymer electrolyte fuel cells. Generally, the platinum-carrying carbon black is prepared by adding sodium hydrogen sulfite to an aqueous acidic platinum-chloride solution, which is then caused to react with hydrogen peroxide to form platinum colloid, supporting the platinum colloid on carbon black, cleaning, and then subjecting the platinum-carrying carbon black to heat treatment as needed.
Platinum, which is an expensive noble metal, has been desired to exhibit sufficient performance even where a small amount of platinum is carried on a carbon support. Therefore, technologies for enhancing the catalyst activity with a reduced amount of platinum used have been studied. For example, an electrode catalyst for fuel cells as disclosed in Japanese Patent Application Publication 2002-289208 (JP-A-2002-289208) is composed of an electrically conductive carbon material, metal particles that are supported on the conductive carbon material and are less likely be oxidized than platinum under acidic conditions, and platinum that covers outer surfaces of the metal particles. The thus formed electrode catalyst has high durability, and the growth of platinum particles during operation of the fuel cell is suppressed or restricted. More specifically, examples of the metal particles include, for example, alloys comprised of platinum and at least one metal selected from gold, chromium, iron, nickel, cobalt, titanium, vanadium, copper, and manganese.
In order to improve the utilization factor of noble metal and reduce the amount of noble metal used so as to reduce the cost of manufacture of electrode catalysts, an electrode catalyst as disclosed in Japanese Patent Application Publication No. 2005-25947 (JP-A-2005-25947) is prepared by subjecting highly dispersed carbon having a highly complex structure and a low specific surface area to a process selected from an activation process, oxidation process using nitric acid, a process for making the carbon hydrophilic, and a hydroxyl addition process, so as to activate the surface of the carbon, and then depositing metal on the carbon.
However, the platinum catalyst or platinum-alloy catalyst of the related art is still insufficient in terms of the oxygen reduction capability, and it has been desired to develop higher-performance catalysts.